The present invention relates generally to superconducting electronic devices, and more specifically, to integrated drive and readout circuits for superconducting qubits.
The fundamental element of a quantum computer is the quantum bit which is known as the “qubit.” As opposed to a classical bit representing zero and one, a qubit is also able to represent a quantum superposition of the two states. The states can be formalized within the laws of quantum physics as a probability of being in the two states. Accordingly, the states can be manipulated and observed within the laws of quantum physics.
In cavity quantum electrodynamics, quantum computing employs nonlinear superconducting devices (i.e., qubits) to manipulate and store quantum information at microwave frequencies, and resonators (e.g., as a two-dimensional (2D) planar waveguide or as a three-dimensional (3D) microwave cavity) to read out and facilitate interaction among qubits. As one example, each superconducting qubit can include one or more Josephson junctions shunted by capacitors in parallel with the junctions. The qubits are capacitively coupled to resonators such as, for example, 2D or 3D microwave cavities).
The electromagnetic energy associated with the qubit is stored in the Josephson junctions and in the capacitive and inductive elements forming the qubit. In one example, to read out the qubit state, a microwave signal is applied to the microwave readout cavity that couples to the qubit at the cavity frequency corresponding to the qubit state. The transmitted (or reflected) microwave signal goes through multiple thermal isolation stages and low-noise amplifiers that are required to block or reduce the noise and improve the signal-to-noise ratio. The microwave signal is measured at room temperature. The amplitude or phase of the readout microwave signal (or both) can, depending on the readout scheme, carry information about the qubit state. This readout signal can be measured and analyzed using room-temperature electronics. Microwave readout provides a stable signal amplitude for control, and commercial off-the-shelf (COTS) hardware is available to use.
Quantum systems such as superconducting qubits are very sensitive to electromagnetic noise, particularly in the microwave and infrared domains. In order to protect these quantum systems from microwave and infrared noise, several layers of filtering, attenuation, and isolation are applied. Particular interest is directed to the layers of protection employed on the input and output (I/O) lines, which are also called transmission lines. The I/O lines (transmission lines) are connected to the quantum system and carry the input and output signals to and from the quantum system respectively. In the case of superconducting qubits, these I/O lines (transmission lines) are usually microwave coaxial lines or waveguides. Some of the techniques or components that are used in order to block or attenuate the noise propagating or leaking into these transmission lines are attenuators, circulators, isolators, low-pass microwave filters, bandpass microwave filters, and infrared filters which are based on lossy absorptive materials or dispersive elements. An integrated drive and readout circuit is needed to drive and readout the superconducting qubits with a minimum number of input and output transmission lines and minimum number of components.